This invention relates to an antenna apparatus, and more particularly, to an antenna apparatus, for example, in the antenna apparatuses used for telecommunications or a radar, in which beam formation is performed by arranging a plurality of element antennas.
FIG. 7 is a diagram showing the construction of a conventional antenna apparatus, e.g., one described in Japanese Patent Application Laid-open No. Hei 7-288417. In the figure, reference numeral 1 designates a plurality of element antennas arranged on a plane and reference numeral 2 designates concentric circles (or concentric circumferences) along which the element antennas are arranged. A feeder means (not shown), which adjusts the excitation amplitude and the excitation phase, is connected to each element antenna 1.
The operation will next be described. This antenna apparatus can have desired radiation characteristics by adjusting the excitation amplitude and the excitation phase with respect to each element antenna 1 by the feeder means.
The conventional antenna apparatus thus arranged has a problem in that if the spacing between the element antennas 1 in a circumferential direction along each concentric circle 2 is increased, high-level sidelobes are generated and the desired radiation characteristic cannot be obtained.
The element antenna spacing may be reduced to avoid such sidelobes. However, if the spacing is reduced to a value smaller than necessary, the number of element antennas is increased and an increase in cost results. Moreover, a problem arises in that mutual coupling between the element antennas is increased and it is therefore difficult to obtain the desired radiation characteristic.
This invention has been achieved to solve the problems described above, and an object of this invention is to provide a low-cost antenna apparatus having the minimum number of element antennas required to suppress unnecessary sidelobe levels.
The present invention relates to an antenna apparatus in which a plurality of element antennas are arranged on a plurality of concentric circles assumed to exist on a plane and differing in radius from each other, and which forms a beam in a direction inclined by xcex80 at the maximum from a direction perpendicular to the plane, said antenna apparatus being characterized in that if the radius of the nth concentric circle from the inner side is an; the number of element antennas arranged on the nth concentric circle from the inner side is Mn; and the number of waves is k, the number Mn, of element antennas arranged on each concentric circle is determined so as to satisfy the following equation:
Mn+0.81xc2x7Mn1/3 greater than kxc2x7anxc2x7(1+sin xcex80)
and in that the element antennas are arranged on each concentric circle by being generally equally spaced apart from each other in the circumferential direction of the concentric circle.
Further, in the apparatus, it is configured such that if the radius of the innermost concentric circle is a1; the number of element antennas existing on the circumference thereof is M1; the radius of the n-th concentric circle form the inner side is na1; and the number of element antennas existing on the circumference thereof is nM1; the number M1 of element antennas existing on the innermost concentric circle is determined so as to satisfy the following equation:
xe2x80x83M1+0.81xc2x7(M1/n2)1/3 greater than kxc2x7a1xc2x7(1+sin xcex80)
Further, in the apparatus, it is configured such that the number Mn of element antennas arranged on the nth concentric circle from the inner side is set to an odd number.
Further, in the apparatus, it is configured such that the number M1 of element antennas arranged on the innermost concentric circle is set to an odd number.
Further, in the apparatus, it is configured such that with respect to an imaginary straight line passing through the center of the plurality of concentric circles, the element antennas on the concentric circles are arranged so as not to be aligned on any straight line parallel to the imaginary straight line.
Further, in the apparatus, it is configured such that the element antennas arrangement start position on each concentric circle has an angular displacement through an randomly selected angle of xcex94n from a straight line passing through the center of the concentric circles.
Further, in the apparatus, it is configured such that with respect to an imaginary straight line passing through the center of the plurality of concentric circles, the number of element antennas existing on one side of the straight line and the number of element antennas existing on the other side of the straight line are made approximately equal to each other.
Further, in the apparatus, it is configured such that feed to the plurality of element antennas is performed by means of a radial waveguide.